This invention relates to a rechargeable alkaline battery, and more particular this invention relates to an arrangement of separators for a silver-zinc alkaline rechargeable battery.
An attractive feature of zinc alkaline batteries and particularly silver-zinc batteries is their high power density. They possess one of the highest gravimetric and volumetric energy densities of commercially available batteries. Additionally, they possess low self-discharge rates as well as high current discharges upon demand.
Despite these advantages, a number of limitations have prevented the more widespread use of these batteries. These batteries have suffered from short cycle life, lasting less than 50 cycles when subjected to field conditions with infrequent cycling. The observed degradation of capacity is due to a combination of several factors. First, there is a change of the scope of the anode electrode due to excessive zinc solubility. Secondly, there is separator degradation via silver migration and plating. Thirdly, there is zinc migration through the separator, leading to premature localized shorts due to the presence of dendrites. Fourth, and final, there is separator degradation by electrolyte.
One technique dealing with zinc migration involves physically diminishing the presence of zinc in critical areas where dendrite formation is not wanted. Thus, Colburn in U.S. Pat. No. 6,153,328 describes a metal/air fuel cell which contains dendrite elimination zones characterized by the substantial lack of zinc in the volume of the zone and substantial presence of a cell positive electrode through the volume.
Adding complexing agents to the zinc electrode or adding agents to the electrolyte suppresses zinc crystal growth. Lian et al in U.S. Pat. No. 5,830,601 teach an electrolyte active species comprising a metal hydroxide such as KOH or NaOH, and a modifier such as a metal porphine, and/or a polyvinyl resin such as polyvinyl alcohol or polyvinyl acetate to suppress zinc deposition. Kawakami, et al in U.S. Pat. No. 5,824,434 use a multi-layered oxide film next to the zinc. U.S. Pat. No. 5,681,672 by Lee teaches adding barium fluoride to the electrolyte solution as a remedy for zinc dendrites. Adler, et al in U.S. Pat. No. 5,302,475 use an electrolyte containing KOH and a combination of KF and K2CO3 salts. Ando in U.S. Pat. No. 4,479,856 adds a quaternary ammonium salt and at least two metal ions selected from the group consisting of lead, tin and cadmium ions as dendrite inhibitor to the electrolyte.
Another approach involves changing the electric field experienced by zinc. U.S. Pat. No. 5,780,186 by Casey Jr. discloses a specially treated zinc that fills the pores of a porous metal substrate. The use of indium sulphate as a coating agent for zinc is disclosed in U.S. Pat. No. 5,626,988 by Daniel-Ivad, et al.
Modifying the separators has been disclosed to prevent zinc dendrites. U.S. Pat. Nos. 4,154,912 and 4,272,470 disclose crosslinking of polyvinyl alcohol by acetalization which supposedly forms networks between polymer molecules, thereby impeding zinc migration. In U.S. Pat. No. 6,033,806, Sugiura, et al discuss a similar crosslinked polyvinyl alcohol separator formed by adding an oxidizing agent to effect oxidative cleavage of 1,2-diol units and then acetalizing to form a film of crosslinked polyvinyl alcohol. Pemsler, et al in U.S. Pat. No. 4,592,973 disclose a separator with a hydrophobic, microporous membrane whose pores are filled with a liquid transport agent comprising an organic agent dissolved in an organic solvent. A zinc-dendrite resistant separator made from copolymers of ethylene and acrylic or methacrylic acid is disclosed in U.S. Pat. No. 4,434,215 by Wszolek, et al. Shibley et al disclose in U.S. Pat. No. 4,371,596 a separator made from a porous, flexible substrate coated with an alkaline insoluble thermoplastic rubber-based resin and an alkaline reactive polar organic plasticizer along with polar particulate filler materials. A separator comprising thermoplastic rubber, an inert filler and a processing agent selected from stearic acid, stearic acid metal salts, stearic acid amides, and natural or synthetic waxes is taught in U.S. Pat. No. 4,327,164 by Feinberg et al.
There have been various attempts to modify separators, to control silver plating, as for example in the chemical modification of the surface of regenerated cellulose. In U.S. Pat. No. 5,763,557, Sanduja et al graft a polymer on the surface of a film of regenerated cellulose by contacting the film with a solution of silver nitrate and an alkali-metal hydroxide. This is followed by contacting the surface with a polymerizable monomer and a catalyst. The polymerization takes place directly on the surface molecules of the substrate. Similar techniques are used in U.S. Pat. No. 5,342,659. Similarly, U.S. Pat. No. 4,272,470 discloses crosslinking polyvinyl alcohol resin with a polyaldehyde-polysaccharide crosslinking agent that will confer resistance against degradation.
Another approach to impart resistance against silver degradation has been to add wettability to the surface of a film that would otherwise be hydrophobic. Thus, for example, U.S. Pat. No. 4,253,927 discloses modifying the surface of polyethylene film by grafting carboxyl groups onto the surface of the polyethylene film. Carboxyl groups as carriers for ionic charge through the separator material. U.S. Pat. No. 5,426,004 recommends the use of alternating layers of a low density, radiation-grafted polyethylene layer provided adjacent the silver electrode and a regenerated cellulose film layer provided adjacent the polyethylene layer. Additional alternating layers of polyethylene and cellophane film can be used. Degradation resistance is a natural consequence as long as the degree of grafting is a small fraction of the bulk polymer.
The present invention controls zinc dendrite formation and metal deposition or plating from metal ions released by the counterelectrode by forming a stack of cellulose separators containing selected, particulate salts that control the concentration of zinc ions and metal cathode ions in the battery cell. The salt-filled cellulose separators are reinforced by the presence of the particulate fillers and are found to be mechanically strong and to have superior ionic transport properties as compared to polyolefin-based separators.
The regenerated cellulose separators utilized in the invention are preferably recombinant separators having dispersed hydrogen permeable domains such as the separators disclosed in co-pending application Ser. No. 09/839,276 entitled RECOMBINANT CELLULOSE SEPARATORS, the disclosure of which is expressly incorporated herein by reference. The domains preferable comprise a cellulose ether containing a C2-C10 ether group such as ethylcellulose. The separator is formed by dissolving cellulose in a first solvent and dissolving a hydrogen gas permeable cellulose ether in a second solvent. The solutions are combined and pre-selected metal salt particles are dispersed in the solution. The solvent is removed to form a separator film ultimately having a thickness from 10 microns to 250 microns.
A zinc alkaline battery according to the invention includes a container, preferably a thin, flat 2-part enclosure containing a negative electrode comprising zinc or zinc oxide, suitably dispersed in a gel or polymeric carrier or binder and a positive electrode such as silver oxide. The electrodes can be mounted on conductive supports such as expanded metal or metal screens suitably formed of silver or coated with a layer of silver, copper. Lead, indium, tin or other corrosion resistant metals. The electrodes are separated by a stack of regenerated cellulose films according to the invention.
A separator film facing the cathode preferably has a dispersion of a metal salt that prevents deposition of silver oxide. The preferred salts are metal fluorides that have limited solubility. The fluoride ion slowly dissolves and reacts with silver ions to form a soluble silver fluoride salts that retard or prevent silver deposition on the separator.
An additional separator may optionally be present in the stack that contains a salt of a metal ion that reacts with hydroxyl groups on cellulose and prevents zinc from reacting with the same groups. A copper salt is the preferred moiety as copper is higher in the electromotive series than zinc. This separator contributes to a substantial decrease in migration of zinc from the zinc electrode to the cathode. However, excessive copper can be leached from the separator film and plate on the zinc electrode. Another optional separator film in accordance with the invention contains a dispersion of particles of a salt such as a sulfide that reacts with the copper ions to form an insoluble salt that precipitates. The sulfide containing separator film is preferably isolated from direct contact with the anode by placing a hydrogen-permeable regenerated cellulose separator film between the anode and the sulfide-containing separator.
The stack of cellulose layers can contain additional layers such as at least one hydrogen permeable layer of a hydrocarbon resin such as polyethylene or polypropylene between any of the salt-filled layers, generally from 1 to 5 layers of 50 micron grooved polypropylene such as Celgard. The Celgard further limits zinc ion migration. Additional hydrogen permeable regenerated cellulose layers may be present such as 1-5 layers of recombinant regenerated cellulose not containing a dispersion of salt.
These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.